The present invention relates to a DC-DC converter, and more particularly, to a control circuit for a DC-DC converter.
In the prior art, electronics devices, such as personal computers, use DC-DC converters as power supplies. Electronics devices are required to reduce power consumption. Accordingly, DC-DC converters are required to reduce power consumption.
In a switching DC-DC converter of the prior art, a first output MOS transistor is turned on so that energy is supplied from its input to its output. The first MOS transistor is turned off so that energy accumulated in an inductor is discharged. When the energy is discharged, the forward voltage of a rectifier diode drops. As a result, some of the energy accumulated in the inductor is lost.
To prevent this, the DC-DC converter includes a second MOS transistor, in lieu of the rectifier diode, and further includes a comparator, which is connected to the source and drain of the second MOS transistor. The comparator detects the current flowing through the inductor based on the potential difference between the two terminals of the second MOS transistor and turns the second MOS transistor on and off based on the detection result. When current flows from a load to ground via the inductor, the second MOS transistor is turned off in response to an output signal of the comparator. This prevents the efficiency of the DC-DC converter from decreasing in a low load state.
U.S. Pat. No. 4,349,776, Japanese Laid-Open Utility Model Publication No. 4-101286, Japanese Laid-Open Patent Publication No. 6-303766, U.S. Pat. No. 5,912,552, and Japanese Laid-Open Patent Publication No. 10-225105 describe the above structure.
The above structure is also found in Leo Francis Cassy, “Circuit Design for 1-10 MHZ DC-DC Conversion”, Massachusetts Institute of Technology 1989, January 1989, in “PFM and PWM synchronous rectification step-down regulator”, FIND, Fujitsu Limited, 2003, Vol. 21, No. 5, pp. 45-47, and in “1-channel PFM and PWM synchronous rectification step-down DC-DC converter IC”, FIND, Fujitsu Limited, 2004, Vol. 22, No. 6, pp. 28-31.